Dolby Digital 5.1:

With Dolby Digital 5.1 you have the top of technology as it stands today! There is discrete channel soundtracks encoded to the AC-3 bit stream so that the sound is heard through the five main speakers and a sub-woofer. Each speaker is independent of the others and has it's own track. The result is the greatest degree of realism that can currently be found. This audio format comes in two basic varieties, Dolby 5.1 and DTS Six Channel. Recently, there is a variation with seven channels, adding a rear center speaker.  Even if a DVD is listed as 5.1 there can be a lot of variation in the actual use of all the speakers. For example, some films rely more on the use of the rear speakers, some may hardly use the sub woofer. It all depends on the decisions made during the mixing of the audio tracks.